For over three decades, the electron microscope has been an integral part of the research program of the Epidemiology Section of the Laboratory of Infectious Diseases regarding the etiologic role, natural history and physical characteristics of various important enteric viruses. Past highlights of these investigations have included inquiry into the etiology of diseases of unknown cause involving studies of fastidious agents of disease that defied cultivation in any cell culture system. The electron microscope played a major role (i) in the discovery of the 27nm Norwalk virus (now known as a member of the Norovirus genus in the calicivirus family) and the establishment of its etiologic role in human diarrheal disease, and (ii) in collaboration with the Hepatitis Viruses Section, in the discovery of the 27nm hepatitis A virus particle and establishment of its etiologic role in infectious hepatitis. It also was used as an important epidemiologic tool for the detection of rotaviruses in stools of infants and young children hospitalized with diarrhea. Although intensive efforts have been made by numerous groups of investigators, the Norwalk and related human enteric noroviruses have yet to be cultivated in any tissue culture system. This, along with the expression of 27nm virus-like particles of some members of this group of agents, as well as studies of expressed viral proteins of rotavirus, have contributed to the continuing important role of the electron microscope in the research program of the Epidemiology Section. The electron microscope continues to be the only method (i) for direct detection of the noroviruses and derivative 27nm expressed virus-like particles and (ii) for unraveling the antigenic relationships among these fastidious agents. In addition, its role in attempting to visualize the presence of expressed rotavirus virus-like particles and to discern their morphology continues to be relevant to the research program.